Vol.3, No.4, 319-322 (2011) Natural Science
http://dx.doi.org/10.4236/ns.2011.34041
Study of the second harmonic emission of
glycine-sodium nitrate crystals at different pH
Ramon Antonio Silva-Molina1*, Mario Enrique Alvarez-Ramos1, Erasmo Orrantia-Borunda2,
Judith Parra-Berumen2, Esperanza Gallegos-Loya2, Enrique Torres-Moye2, Daniel Lardizabal2,
Alberto Duarte-Moller2
1Universidad de Sonora, Sonora, México;
*Corresponding Author:
2Centro de investigación en materiales avanzados S.C., Chihuahua, México
Received 16 December 2010; revised 13 January 2011; accepted 9 February 2011.
ABSTRACT
This work shows an optical and structural study
of glycine sodium nitrate crystals. This study
was supported with the respective X ray diffract-
tion and Second-Harmonic Generation signal
detection by using a little variant to the Kurtz-
Perry method. The goal of this work is to obtain
the right pH that modifies the charge of glycine
sodium nitrate system in order to obtain the
best second harmonic emission. Furthermore,
with the change on the charge on the aminoacid,
it is observed how it modifies the optical prop-
erties in the glycine sodium nitrate complex.
Keywords: RAMAN; GSN; NLO; SHG; pKa
1. INTRODUCTION
Nonlinear optical (NLO) materials have wide applica-
tions in the area of laser technology, optical communica-
tion and electro optics application. The nonlinear optical
effect is the interaction of an electromagnetic field of
high intensity laser light with a material [1-3]. The de-
velopment of photonic and optoelectronic technologies
rely heavily on growth of NLO materials with the high
light no linear responses. A NLO material need to have a
large NLO coefficient, large birefringence, wide trans-
parency range, high damage threshold, broad spectral
and temperature bandwidth, good chemical and me-
chanical stability, ease of growth and low cost [4-6]. One
of the advantages in working with organic materials is
that they allow one to fine-tune the chemical structures
and properties for the desired nonlinear optical proper-
ties. Additionally, they have large structural diversity.
The properties of organic compounds can be refined
using molecular engineering and chemical synthesis [6].
The second harmonic generation SHG is a phenomenon
produced by the second order nonlinearities in a material
when it is exposed to high intensity and monochromatic
light source. Given glycine amino acids as an ampho-
teric, it can be assumed as cationic, anionic and zwit-
terionic configurations, i.e. the charge distribution is
determined by the pH and the pKa of the carboxylic
group (pKa = 2.34) and the amino group (pKa = 9.6).
Thus, in the pH range between 2.34 and 9.6, most of
molecules are zwitterionic with both ends charged NH3+
and COO− [1,7,8]. Additionally the study of the effect on
the pH on the complex GSN can provide the correct way
to rowth crystals based on aminoacids with a good mor-
phological quality and excellent optical properties.
2. EXPERIMENTAL
2.1. Crystal Growth
The GSN crystals were obtained by using 99.9% purity
Sigma Aldrich glycine (NH2-CH2-COOH) with FW =
75.57 g/mol, and Sigma Aldrich sodium nitrate (NaNO3)
(99.9%) with FW = 4.99 g/mol. A stoichoimetric mix-
ture of glycine and sodium nitrate in equimolar ratio was
dissolved in 100ml of water distillated with stirrer mag-
netic in a thermoplate. In order to modify the charge of
GSN molecule, seven samples with different pH (1,3,4,
7,9, 10,11) were prepared [10]. In this sense we have ob-
tained three electric glycine configurations (Zwitterionic,
Cationic, and Anionic). The pH was adjusted with nitric
acid concentrate HNO3 and ammonium hydroxide
NH4OH. As follow step the crystals ware retired of the
solution are watched with distillated water and immedi-
ately drying to prevent clusters formation, crystalline
inclusions and eliminate impurities on surfaces. Hence,
the size and quality of crystals dependent on the molar
ratio in the reagents compared with the solvent, i.e. for
low concentrations, crystals are big and for high concen-
trations, crystals are small.

R. A. Silva-Molina et al. / Natural Science 3 (2011) 319-322 320
3. CHARACTERIZATION
3.1. Crystal Growth
GSN crystals were obtained by a slow evaporation
technique for aqueous solutions. The crystals were pre-
pared with distilled water containing glycine, Sodium
Nitrate [NaNO3] in molar ratio 1:1 with a starting pH of
6.4, and then changing the pH of the solution at 1,3,
4,7,9,10,11.
Transparent crystals of different size and shapes were
obtained in about two to three weeks at room tempera-
ture. The size of the crystals was found to be depending
on the amount of material available in the solution which
in turn is decided by the solubility of the material in sol-
vent. The shapes were found to be determined by the pH
of the solution. Figure 1 displays the micrographs of
crystals grown at different pH.
3.2. RAMAN Spectroscopy
The RAMAN spectroscopy is a powerful technique
used for the analysis of organic compounds which is
useful for any state of matter and especially in biological
samples. Other advantage of RAMAN spectroscopy is
the use of visible radiation, this allows narrow down the
warming effects in the sample[11]. The RAMAN spectra
can be identified as roto-vibrational spectra, because the
lines of RAMAN frequency correspond to the distance
between energy levels. Hence, the main transitions are
due to the normal vibrational modes and determinate the
modes that change the polarization in the molecule, this
characteristic is the main reason why RAMAN is useful
in the analysis of GSN [12]. In the present work the
RAMAN spectra was carried out at room temperature in
frequency range 400 - 4000 cm−1 with Xplora RAMAN
microscope HORIBA system.
The Figure 2 shows the symmetric and asymmetric of
the functional group NH3+ and the stretching vibrations
found in 3244 y 2884 cm−1 frequency. Furthermore, the
Figure 1. Single crystals of GSN recrystalized at different pH.
Figure 2. The RAMAN spectra of GSN at different pH.
position and broadness of this mode, 3
NH asymmetric
stretching frequency, indicate the formation of both, in-
tra and intermolecular strong N-H—O hydrogen bonding
of the 3
NH

group, with the oxygen of both, the carbonyl
group and inorganic nitrates. Hence, the presence of this
bonds make what are found lowering frequencies 2884
cm−1 [4,13]. The crystal structure of GSN show that the
organic molecular units are located between layers of
NaNO3 chains and linked to sodium nitrate by strong
intramolecular hydrogen bonds of N-H—O type. This
structural organization of infinite chains of highly polar-
ity entities connected in a head to tail arrangement in
GSN is behalf in contribution to the NLO properties of
the crystal. The study of symmetry and stretching vibra-
tion of CH2 group is observed around 3023 and 2969
cm−1. The CH and NH bending observed in 1616 and
1510 cm−1 frequency. The absorption peaks at 2009 and
1615 cm−1 confirmed the presence of 3
NH

bending.
The peak at 1408, 586 and 509 cm−1 is assigned to the
symmetric stretching C-COO carboxyl group. The band
around 1118 cm−1 is also indicative of the NH3 rocking
modes. The band around178 cm−1 it is indicative of tor-
sion of Na. The wavelength was observed and the pro-
posed allocation of spectrum is shown in the following
Table 1.
3.3. X-Ray Diffraction
In order to obtain the structural parameters of the
crystal under study, we also achieve a powder X ray dif-
fraction to confirm the phase. The analysis of the ob-
served spectra was performed using X’Pert data collector,
powder diffraction data interpretation and indexing
software program X’Pert Highscore Plus. Version 2.2a.
The XRD peaks were indexed and the unit cell was
found to have monoclinic symmetry with cell parameters

R. A. Silva-Molina et al. / Natural Science 3 (2011) 319-322 321
Table 1. Raman assignment.
Frequency RAMAN/cm−1 Assignment
3243 3
NH Asym Strech
3024 CH2 Asym Strech
3000 ?-Glycline
2976 CH2 Sym Strech
2884 N-H…0 Sym Strech
2725 overtones
2616 overtones
1659 Overtones
1614 3
NH Asym Bend
1508 3
NH Sym Bend
1448 CH2 Scissoring
1397 3
NO Asym Strech
1329 CH2 Wagging
1309 CH2 Wagging
1143 CH2 Twisting
1114 3
NH Rocking
1052 3
NO  Sym Strech
939 CH2 Rocking
895 C-C Strech
723 COO− Deform
677 3
NO inplane Deform
588 COO− Deform
508 COO− Rocking
398 3
NH Torsión
330 CCN Bending
178 Na+ Translation
138 COO− Torsion
109 N…O Vibrations
a = 14.326 Å, b = 5.261 Å, c = 9.115 Å, β = 119.070 and
unit cell volume of 600.45 A3. The Figure 3 showed that
basic pH obtained the major phase of GSN com- pared
with acid pH. This is because, as the pH becomes more
acid, the diffraction patterns show that it reduces the
phase of GSN and other compounds are generated.
Similar information has been reported for the authors
elsewhere [14] in the L-alanine sodium nitrate. Also a
slow overtone signal in the NH3 group is characteristic
of the nonlinear emission.
Figure 3. The X-ray pattern for GSN at different pH.
Figure 4. The SHG signal of GSN at different pH.
3.4. Second-Harmonic Generation
Second-harmonic generation (SHG), or frequency
doubling, can be defined as the conversion of a specific
wavelength of light into half its original λ11/2 λ1, or
with respect to frequency ω, ω12 ω1. A tipical setup
for power SHG measurements is made for modified
Kurtz- Perry method. Also, a low energy laser, pulsed or
continuous, is needed. [2,14,15] Usually Nd-YAG laser
(1064 nm output) is used and the sample is a polycrys-
talline powder. With normal size of 70 μm each crystal,
is shown the SHG measurements with respect to differ-
ent pH of GSN from 1 to 11. Figure 4 shows the effi-
ciency of GSN samples at different pH.
The SHG efficiencies are pH 3 this due is closer to
pKa = 2.3 of glycine and the dipole moment is majorly
due to the change of charge of the molecule, however the

R. A. Silva-Molina et al. / Natural Science 3 (2011) 319-322 322
change in the dipole moment of the molecule above of
pKa = 9.7 also shows a good efficiency, it is given that
the sample with more acid pH showed that contains
γ-glycine and the more basic pH showed that contains
GSN phase in more concentration.
The transparent glycine sodium nitrate crystals (GSN
crystals) were successfully obtained using slow evapora-
tion technique at room temperature and we characterized
them by various techniques. The presence of fundamen-
tals groups was verified by a RAMAN microscope. The
GSN structure was characterized using XRD powder, the
X-ray pattern showed that the samples of GSN to basic
pH contained the GNS phase and the more acid pH is
observed that is obtained GSN on minor concentration
but too obtain subproducts like γ-glycine which increase
the efficiency of SHG.
4. ACKNOWLEDGEMENTS
The authors thank to the National Council of Science and Techno-
logy of Mexico for its financial support, (grant 132856). Also to the
Center for Research on Advanced Materials, Department for Polymers
and Materials Research and Department of Physics.
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